Lymphocyte transformation in experimental symphilis: Rabbits infected with T. pallidum, nichols strain were used for this experiment. The H3 thymidine incorporation technique was used to examine the in vitro leukocytes stimulation to Reiter and VDRL antigen. Wassermann and antitreponemal antibodies were also examined. The results of the stimulation were expressed as stimulation indices. An inhibition of stimulation, as compared to the pre-infection value was observed until the 24th day of infection. The peak of stimulation was reached on day 44 and one week later the stimulation indices declined to pre-infection values. The stimulation indices to PHA remained unchanged until the 34th day of infection, increased significantly on day 44 and returned to pre-infection values on day 50th after infection. No correlation between the Wassermann or antitreponemal antibodies and Reiter or VDRL antigen stimulated cultures was observed. The unexpected phenomenon is under further examination. Migration inhibitory effect of cells from rabbits infected with T. pallidum: The direct macrophage inhibitory factor (MIF) agarose assay used. T. pallidum, Reiter protein and VDRL were used as antigens. Cells of rabbits infected for various lengths of time were examined. The range of leukocytes migration inhibition in the presence of the treponemal antigen in rabbits infected from one week until one year, varied between 5-20%. The leukocytes migration inhibition in the presence of Reiter antigen, fluctuated between 5-10%. No inhibition of the leukocytes migration in the presence of the VDRL antigen was observed.